ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Recent Posts
Hello... again!
by HotLine1 - 07/15/24 02:47 PM
90.5(C) Explanatory Material 2023
by gfretwell - 07/15/24 01:19 PM
2023 CEU Course
by renosteinke - 07/12/24 03:26 PM
Counterfeit Code Books?
by Comprehensiveedu - 07/12/24 12:16 PM
Old low volt E10 sockets - supplier or alternative
by gfretwell - 07/11/24 08:51 PM
New in the Gallery:
This is a new one
This is a new one
by timmp, September 24
Few pics I found
Few pics I found
by timmp, August 15
Who's Online Now
1 members (Scott35), 458 guests, and 15 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 3 1 2 3
#191085 12/17/09 09:58 PM
Joined: Jan 2005
Posts: 354
pdh Offline OP
Equipment needs 208 volts single phase. Supply is only 480Y/277. So a transformer is needed. How would you wire it?

Stay up to Code with the Latest NEC:

>> 2023 NEC & Related Reference & Exam Prep
2023 NEC & Related Reference & Study Guides

Pass Your Exam the FIRST TIME with the Latest NEC & Exam Prep

>> 2020 NEC & Related Reference & Study Guides

pdh #191092 12/17/09 11:39 PM
Joined: May 2005
Posts: 984
Likes: 1
First I'd see which single-phase transformer was cheaper and more readily available: 277V to 208V or 480V to 208V.
Then feed it with either a 1-pole or 2-pole C/B respectively.

You'll also need an OCP on the 208V side of the transformer.

Joined: Jan 2005
Posts: 354
pdh Offline OP
How would you WIRE it? Where would the ground come in to prevent it from being an ungrounded system?

pdh #191094 12/18/09 12:35 AM
Joined: Feb 2003
Posts: 939
You only need to bring out EGC due you have line to line load that will be bonded to the transfomer.

If this is line et netural load then everything will change a bit.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Jul 2004
Posts: 9,950
Likes: 34
You would still have to ground it somewhere on the secondary. The easiest way is to make one leg a neutral by grounding it. I would use gray for that neutral and orange for the hot, just to wake up the next guy that there is something strange going on here. (similar to the gray/violet on the 277)
The only problem with that idea is orange is one of the BOY colors and you have 480 there.

Greg Fretwell
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and

There are (3) ways to go on this scenario - all are based on what is acceptable for the Equipment involved.


This option is the most simplest: Tapped Autotransformer.

Using a 277V wound Autotransformer, with a tap for 208V:
  1. Connect the Transformer across a 277V 1 Phase 2 Wire "L-N" Circuit.
  2. Place the Grounded Conductor on the "Common" side of the Autotransformer.
  3. From the 208V Tap, bring out an Ungrounded Conductor.
  4. From the "Common" side connection, bring out the Grounded Conductor.
  5. From numbers "3" and "4" above, a 208V 1 Phase 2 Wire Circuit is created.

Insert OCPDs as necessary. At minimum, OCPD for the Primary side Feeder.
Include the EGC with the Primary Circuit, and bring it out for the 208V Secondary Circuit.
Do not bond the Grounded Conductor!!!



Single Phase Isolated Transformer with 208V Secondary.

As mentioned by Ghost307, use an Isolated transformer with a Primary Voltage rating that is readily available.

Feed the Primary side from the available System:

* For a 277V Primary: One Ungrounded Conductor, plus the System Grounded Conductor.

* For a 480V Primary: Two Ungrounded Conductors.

Run the EGC to the Transformer case; bond it to the case.

On the Secondary side, bond Terminal "X2" to a local Grounding Electrode System.
Additionally, install a Jumper from "X2" to the Transformer case.

Derive an SDS from the Secondary side for the new 208V 1 Phase System as follows:

  1. From Terminal "X1", bring out the 208V System's Ungrounded Conductor.
  2. From Terminal "X2", bring out the 208V System's Grounded Conductor. Identify this Conductor as a System Grounded Conductor, using either White or Gray Coloring.
  3. From the "Jumper" at Terminal "X2", bring out an EGC. Identify with Green Coloring.

The Secondary Feeders _MAY_ be protected by the Primary Feeder OCPD, or you may choose Over Current Protection for both the Primary Feeders and the Secondary Feeders.

Since the output Voltage is only a single Voltage (208V only), the Secondary Feeders may be protected by the Primary Feeders' OCPD as described below:

A: If the Primary rating is 9 Amps or more: Maximum OCPD on Primary side is 125% the FLA of the Primary Winding(s).

B: If the Primary rating is less than 9 Amps: Maximum OCPD on Primary side is 167% the FLA of the Primary Winding(s).

C: If the Primary rating is less than 2 Amps: Maximum OCPD on Primary side is 300% the FLA of the Primary Winding(s).

The option exist to run the Secondary side as an Ungrounded System. This does not eliminate the need of bonding the Metallic Equipment connected to the Secondary side Circuitry to a local GES.
The ONLY difference between a Grounded System -vs- an Ungrounded System is installing a Main Bonding Jumper; or in this case, a "System Bonding Jumper", between the SDS and a local GES.

For more information on Grounded -vs- Ungrounded Systems, refer to the Transformer Schematics posted at the Technical Reference section.



3 Phase Isolated Transformer - 480V x 208Y/120V:

This option would be the most involved, but may be the most cost effective!

This type of Transformer is an off-the-shelf item, readily available from nearly all Wholesalers.
The costs and availability might result equal to a Single Phase setup.

This option allows for future expansion and usage.

Setup would be similar to "OPTION #2" above, with the following revisions:

A: Primary side fed with 480V 3 Phase 3 Wire Circuit + EGC.

B: Primary side Over Current Protection only covers Primary Feeders.

C: Secondary side shall be Grounded. Ground at Terminal "X0".

D: Secondary Feeders to be Protected by OCPDs. Primary OCPD may not protect Secondary Feeders.

E: Secondary side will consist of (3) Ungrounded Conductors, and (1) EGC bonded to the GES and the "X0" Star Point.

F: Secondary side Grounded Conductor _MAY_ be brought out from Terminal "X0" to a Panelboard / Disconnect if wanted.
If the Panelboard or Switch (Disconnect) will be feeding only L-L / L-L-L connected loads, the System's Grounded Conductor does not need to be included - provided the Main System Bonding Jumper exists at the Transformer, and there is a suitable EGC between the Transformer and the Panel or Switch.


As far as "What Would I Do", I would do one of the three options listed above! smile

BTW, Abbreviations used:

* EGC = Equipment Grounding Conductor.

* GES = Grounding Electrode System.

* OCPD = Over Current Protection Device.

* SDS = Separately Derived System.

* L-N = Line To Neutral.

* L-L = Line To Line.

* L-L-L = Line to Line To Line (3 Phase 3 Wire)


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Jan 2005
Posts: 354
pdh Offline OP
Wow, Scott35, lots of detail.

I wanted to understand what people thought of a single phase 208 volt system. In the form of transformers, it is easier to discuss and understand, so that's why I asked in terms of transformers first. But really, I'm looking at this in terms of UPSes/inverters, and how a 208 volt system is formed. I have a choice between two kinds of UPSes for a bunch of computers:

1. Has one output inverter producing 208V directly (can be programmed to one of 200/208/220/230/240 volts, 50 or 60 Hz). Appears to NOT be grounded in any way. Questions to their tech support people get strange answers, but finally conclude it is ungrounded (neither terminal is a grounded conductor). Any relation to ground will be through the inverter circuitry itself. The input is 3 wire and has an L6-30P.

2. Has two output inverters producing 120V directly (can be programmed to one of 100/110/115/120/127 volts, 50 or 60 Hz). Specs also indicate they can be programmed to 120, 180, and 240 degree phase angles (I guess the 240 is there to reverse any sense of phase rotation without having to reverse the conductors). So apparently it can produce any of 173/190/200/208/220/230/240/254 volts L-L output through appropriate combinations of L-G/N voltage and phase angle. The input is 4 wire (A/B/N/G) and has no plug (why not an L14-30P).

I'm leaning toward the latter because it is grounded. The former is lower in cost and was already ordered before I was hired. Normally, when trying to evaluate equipment, vendors bring along a sample unit and often leave it for a few days trial. This isn't practical in this case.

I guess I biased my question when I replied with asking how to ground the transformer secondary. I wondered if someone might suggest to just leave it ungrounded. Would you do that for a bunch of computers with multiple ungrounded UPSes?

pdh #191183 12/20/09 03:47 AM
Joined: Jul 2004
Posts: 9,950
Likes: 34
The problem is not going to be with the computers, they use isolated power supplies anyway that only look at line to line voltage.
Since this is greater than 150v to ground you are not bound by 250.20 so I was probably wrong about having to ground it.
This is strange territory for me. I have seen lots of 208v but it was always 3p wye or CT delta wild legs.

Greg Fretwell
Joined: Jan 2005
Posts: 354
pdh Offline OP
Of course the PSU will see the 208V L-L. But if it's floating, my concern is the phantom voltages that can disrupt interconnections between computers on different UPSes. Just a wee bit of hum on the ethernet connections can ruin their performance.

3pY would work better in the sense that it has to have the ground reference. But UPSes which output 3pY seem to no longer be made with less than 10 kVA. One rack cabinet doesn't need that much power. Not only is there a premium to the cost for 3p, but also for the excess capacity. That, plus a distribution box wired to balance the 2w1p loads across the 3p would be hard to find. 3p is just not commodity enough to be cost effective.

pdh #191196 12/21/09 03:37 AM
Joined: Jul 2004
Posts: 9,950
Likes: 34
There is no connection between the line voltage and the output of the power supply. This is immediately converted to 20kz or so AC and pumped through a toroid transformer that produces the various DC voltages.

These power supples are designed for a global market and I am sure there are countries that do not use grounded line voltage.
Ethernet rejects 60hz hum anyway. What they do have problems with is large ground shift transients from baseplate ground in one machine to another. (tied to the the EGC)
It tends to fry the card and might even start blowing up other stuff hanging on the bus..

The NEC doesn't make you ground one leg but it doesn't prohibit it either.

Greg Fretwell
Page 1 of 3 1 2 3

Link Copied to Clipboard
Powered by UBB.threads™ PHP Forum Software 7.7.5